Antenna systems with common overhead for CDMA base stations

ABSTRACT

Antenna systems are used for transmitting common overhead channels (pilot, sync, and paging channels) over a whole sector while transmitting and receiving unique traffic channels on individual beams in the sector. Each beam in the sector is transmitted at a frequency offset from other beams in the sector. The offset frequency is chosen such that the effect of cancellation of the pilot channel caused by the summing of signals from multiple beams is minimized. Alternative, each beam in the sector can have a time dependent phase offset relative to each other to minimize the effect of cancellation of the pilot channel caused by the summing of signals from multiple beams. System capacity is substantially increased since the number of traffic carrying beams per sector is increased without using more pilot channel PN offsets. Beams are fixed and the same antennas are used for the overhead channels as the traffic channels, obviating the need for complex algorithms and calibration procedures.

[0001] This invention relates in general to CDMA cellular communicationsystems and in particular to methods and apparatus for increasing thecapacity of such systems.

BACKGROUND

[0002] CDMA digital cellular systems are currently in widespread usethroughout North America providing telecommunications to mobile users.In order to meet the demand for transmission capacity within anavailable frequency band allocation, CDMA digital cellular systemsdivide a geographic area to be covered into a plurality of cell areas.Within each cell is positioned a base station with which a plurality ofmobile stations within the cell communicate.

[0003] In general, it is desired to have as few base stations aspossible, since base stations are expensive, and require extensiveeffort in obtaining planning permission, and in some areas, suitablebase station sites may not be available. In order to have as few basestations as possible, each base station ideally has as large a capacityas possible in order to service as large a number of mobile stations aspossible. The key parameters that determine the capacity of a CDMAdigital cellular system are: processing gain, ratio of energy per bit tonoise power, voice activity factor, frequency reuse efficiency and thenumber of sectors in the cell-site antenna system.

[0004] One method of achieving an increase in capacity is to replace awide beam width antenna with an antenna array that allows the formationof a number of narrower beam widths that cover the area of the originalbeam. Referring to FIG. 1, a conventional CDMA communication cell 100 isshown comprising 3 adjacent hexagonal sectors, alpha 102, beta 104 andgamma 106. Each cell comprises an antenna tower platform 120 located atthe intersection of the 3 sectors. The antenna tower platform 120 has 3sides forming an equal-lateral triangle. Each sector has 3 antennas(only antennas in sector alpha 102 shown) a first antenna 114, a secondantenna 116 and third antenna 112 mounted on a side of the antenna towerplatform 120. Each sector also has 3 beams (only beams in sector alpha102 shown) a first beam 108, a second beam 110 and a third beam 112. The3 beams 108, 110, 112 are adjacent with some overlap. The 3 sectorsalpha 102, beta 104 and gamma 106 are identical in structure withrespect to antennas and beams. The signal for a particular user can thenbe sent and received only over the beam or beams that are useful forthat user. If the pilot channel on each beam is unique (i.e. has adifferent PN (pseudo-random noise) offset) within each sector then theincrease in capacity is limited due to interference between reused pilotchannels in different cells.

[0005] An improvement is to use multiple narrow beams for the trafficchannels and transmit the overhead channels (pilot, sync, and pagingchannels) over the whole sector so that the pilot channel is common toall the narrow beams used by the traffic channels in that sector. Thisleads to substantial gains in capacity. For example, a change from asystem with a single beam per sector to a system with 3 beams per sectorwith a common pilot channel yields a 200 to 300% increase in capacity.It is therefore desirable that the pilot channel be broadcast over thearea covered by the original wide beam. A possible arrangement is to usemultiple beams per sector for the traffic channels and transmit theoverhead channels over a separate wide beam antenna covering the wholesector. However, this requires the expense of extra hardware as well asthe calibration and adjustment needed to match the phase of the pilotchannel with the phase of the traffic channels over time andtemperature.

[0006] Another possible solution is to use adaptive antenna arraytechniques to transmit and receive multiple narrow beams for the trafficchannels and transmit the overhead channels over the whole sector on thesame antenna array. However, this requires complex calibration equipmentand algorithms.

[0007] Yet another solution is to use an antenna array that transmitsand receives multiple sectors over fixed narrow beams for the trafficchannels and transmit the pilot channel on the same fixed narrow beams.However, the problem with this approach is that the strength of thepilot channel signal at any point in the sector is determined by thevector sum of all of the pilot channel signals from each beam. Since thepilot channel signals from each beam are coherent, areas where thevector sum of the pilot channel signals is null or severely degradedwill occur. This can result in dropped calls when a mobile stationenters one of these areas.

[0008] There is thus an advantage to provide an antenna array that usesfixed narrow beams for transmitting and receiving the traffic channelson multiple beams and can broadcast the common pilot channel over all ofthe sector using the same antenna array. Furthermore, it would beadvantageous to provide an antenna system that did not require complexcalibration and adjustment to maintain performance over time andtemperature.

SUMMARY

[0009] The invention may be summarized according to a first broad aspectas an antenna system having multiple antennas defining a respectiveplurality of fixed beams that together cover a sector and are connectedto a beam-forming matrix. Transceivers are connected respectively to thebeam-forming matrix to drive the plurality of antennas, with signalscomprising common overhead channels. The signals may be IS-95, IS-2000or any other similar CDMA communications standard designed forterrestrial cellular communications. In accordance with this first broadaspect the transceivers provide transmit frequencies that are slightlyoffset from one another. The offsets are chosen such that undesirableeffects of the signal cancellation are reduced. More particularly, theoffsets are chosen such that the overall system performance isoptimized.

[0010] The invention may be summarized according to a second broadaspect as an antenna system having multiple antennas defining arespective plurality of fixed beams that together cover a sector and areconnected to a beam-forming matrix. The transceivers are connectedrespectively to the beam-forming matrix to drive the plurality ofantennas, with signals comprising common overhead channels. The signalsmay be IS-95, IS-2000 or any other similar CDMA communications standarddesigned for terrestrial cellular communications. In accordance withthis second broad aspect the transceivers provide transmit phases thathave time dependent offsets with respect to one another. The offsets arechosen such that undesirable effects of the signal cancellation arereduced. More particularly, the offsets are chosen such that the overallsystem performance is optimized.

[0011] The invention may be summarized according to a third broad aspectas an antenna system having a digital beam former connected to aplurality of transceivers and a plurality of antennas defining arespective plurality of fixed beams that together cover a sector. Thetransceivers are connected to the plurality of antennas to drive themwith signals comprising common overhead channels. The signals may beIS-95, IS-2000 or any other similar CDMA communications standarddesigned for terrestrial cellular communications. In accordance withthis third broad aspect the transceivers provide transmit frequenciesthat are slightly offset from one another. The offsets are chosen suchthat undesirable effects of the signal cancellation are reduced. Moreparticularly, the offsets are chosen such that the overall systemperformance is optimized.

[0012] The invention may be summarized according to a fourth broadaspect as an antenna system having a digital beam former connected to aplurality of transceivers and a plurality of antennas defining arespective plurality of fixed beams that together cover a sector. Thetransceivers are connected to the plurality of antennas to drive themwith signals comprising common overhead channels. The signals may beIS-95, IS-2000 or any other similar CDMA communications standarddesigned for terrestrial cellular communications. In accordance withthis fourth broad aspect the transceivers provide transmit phases thathave time dependent offsets with respect to one another. The offsets arechosen such that undesirable effects of the signal cancellation arereduced. More particularly, the offsets are chosen such that the overallsystem performance is optimized.

[0013] Advantageously, the ability to use a plurality of fixed beamswith common overhead channels results in a significant increase insystem capacity.

[0014] Other aspects and features of the present invention will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of the specific embodiments of the invention inconjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a diagram of a conventional tri-cellular CDMAcommunication cell modified to show 3 narrows beams in place of thenormal single wide beam per sector;

[0016]FIG. 2A is a diagram of an antenna system of sector alpha of theCDMA communication cell of FIG. 1;

[0017]FIG. 2B is a diagram of an alternative antenna system of sectoralpha of the CDMA communication cell of FIG. 1;

[0018]FIG. 3 is a diagram showing a transceiver of FIG. 2A and 2B ingreater detail.

[0019]FIGS. 4A and 4B are diagrams showing the vector addition ofsignals from the first beam and the second beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] In order to transmit and receive unique traffic channels on eachbeam in a sector while transmitting common overhead channels (pilot,sync, and paging channels) over all of the beams in the sector anantenna system using fixed narrow beams that does not require complexcalibration equipment and algorithms is provided.

[0021] To this end, FIG. 2A shows a conventional antenna system 200 thatis within sector alpha 112. The sectors beta 104 and gamma 106 haveidentical antenna systems. The antenna system 200 defines a first beam108, a second beam 110 and a third beam 112. The three beams 108, 110,112 are radiation/reception patterns formed by a first antenna 114, asecond antenna 116 and a third antenna 118 respectively. The threeantennas 114, 116, 118 are connected to a beam-forming matrix 240 thatmay be, for example, a Butler matrix. The beam-forming matrix 240comprises three bi-directional ports: a first port 242, a second port244 and a third port 246. The input signals of the first port 242, thesecond port 244 and the third port 246 are transmitted on the first beam108, the second beam 110 and the third beam 112 respectively. Thesignals received on the first beam 108, the second beam 110 and thethird beam 112 are the outputs of the first port 242, the second port244 and the third port 246 respectively. The antenna system 200 alsocomprises a first transceiver 220, a second transceiver 222 and thirdtransceiver 224. The first transceiver 220 has an input 226, an output228 and a bi-directional port 252. The second transceiver 222 has aninput 230, an output 232 and a bi-directional port 254. The thirdtransceiver 224 has an input 234, an output 236 and a bi-directionalport 256. The first port 242, second port 244 and third port 246 of thebeam-forming matrix 240 are connected to bi-directional port 252 of thefirst transceiver 220, bi-directional port 254 of the second transceiver222 and bi-directional port 256 of the third transceiver 224respectively.

[0022]FIG. 2B shows another conventional antenna system 202 that may bedeployed within sector alpha 112. The antenna system 202 defines a firstbeam 108, a second beam 110 and a third beam 112. The three beams 108,110, 112 are radiation/reception patterns formed by a first antenna 114,a second antenna 116 and a third antenna 118 respectively. The antennasystem 202 also comprises a first transceiver 220, a second transceiver222 and third transceiver 224. The first transceiver 220 has an input226, an output 228 and a bi-directional port 252. The second transceiver222 has an input 230, an output 232 and a bi-directional port 254. Thethird transceiver 224 has an input 234, an output 236 and abi-directional port 256. The three antennas 114, 116, 118 are connectedto the three respective bi-directional ports 252, 254, 256 of thetransceivers 220, 222, 224. The antenna system 202 also comprises adigital beam former 260 that has a first input 262, a first output 268,a second input 266, a second output 268, a third input 270, a thirdoutput 272, a first beam output 274, first beam input 276, a second beamoutput 278, a second beam input 280, a third beam output 282 and a thirdbeam input 284. The first beam output 274 and input 276 of the digitalbeam former 260 are connected the input 226 and output 228 of the firsttransceiver 220 respectively. The second beam output 278 and input 280of the digital beam former 260 are connected the input 230 and output232 of the second transceiver 222 respectively. The third beam output282 and input 284 of the digital beam former 260 are connected the input234 and output 236 of the third transceiver 220 respectively.

[0023] Although three antennas forming three beams per sector are usedin this example of the preferred embodiment, any number of antennas andbeams per sector greater than one may be used while remaining within thescope of the invention.

[0024] The transceivers 220, 222, 224 of FIGS. 2A and 2B are identicalin design and are described in greater detail with respect to FIG. 3.For ease of description the transceiver shown in FIG. 3 is given thereference number 300. Transceiver 300 has its input 302 connected to aninput of a modulator 306. The modulator 306 has an output 308 that isconnected to a first input 310 of an up-converter 312. The up-converter312 also has a second input 314 and an output 316. The second input 314of the up-converter 312 is connected to an oscillator 318 that may be,for example, a digital frequency synthesizer. The output 316 of theup-converter 312 is connected to an input 344 of a duplexor 340 having abi-directional port 342 connected to the bi-directional port 320 of thetransceiver 300. The transceiver 300 also has an output 322 connected toan output 324 of a demodulator 326. The demodulator also has an input328 that is connected to an output 330 of a down-converter 332. Thedown-converter 332 also has a first input 334 and a second input 336.The first input 334 of the down-converter 332 is connected to anoscillator 338 and the second input 336 of the down-converter 332 isconnected to an output 346 of the duplexor 340. The up-conversion stageof the transceiver 300, comprising the up-converter 312 and oscillator318, are shown as a single stage for convenience. In reality theup-conversion may be done in a plurality of stages. Similarly, thedown-conversion stage of the transceiver 300, comprising thedown-converter 332 and oscillator 338, are shown as a single stage forconvenience. In reality the down-conversion may be done in a pluralityof stages.

[0025] Referring to FIG. 2A, the signals on input 226, input 230 andinput 234 of transceiver 220, transceiver 222 and transceiver 224respectively are digital baseband signals that are transmitted on thefirst beam 108, the second beam 110 and the third beam 112 respectively.The signals on output 228, output 232 and output 236 of transceiver 220,transceiver 222 and transceiver 224 respectively are digital basebandsignals that are received on the first beam 108, the second beam 110 andthe third beam 112 respectively.

[0026] The digital baseband signals on input 226, input 230 and input234 of transceiver 220, transceiver 222 and transceiver 224 respectivelymay be any CDMA standard digital data stream adapted to be received by aplurality of mobile stations (not shown) within the area covered by thefirst beam 108, the second beam 110 or the third beam 112.

[0027] Similarly, referring to FIG. 2B, the signals on input 262, input266 and input 270 of the digital beam former 260 are digital basebandsignals that are transmitted on the first beam 108, the second beam 110and the third beam 112 respectively. The signals on output 264, output268 and output 272 of the digital beam former 260 are digital basebandsignals that are received on the first beam 108, the second beam 110 andthe third beam 112 respectively.

[0028] The digital baseband signals on input 262, input 266 and input270 of the digital beam former 260 may be any CDMA standard digital datastream adapted to be received by a plurality of mobile stations (notshown) within the area covered by the first beam 108, the second beam110 or the third beam 112.

[0029] The frequency of the oscillator 318 in transceiver 222 is chosensuch that the frequency of the output 316 of the up-converter 312 in thetransceiver 222 is a standard IS-95 base station transmit frequency,f_(c). The frequency of the oscillator 318 in the transceiver 220 ischosen such that the frequency of the output 316 of the up-converter 312in the transceiver 220 is f_(c) plus an offset frequency, f_(o). Thefrequency of the oscillator 318 in the transceiver 224 is chosen suchthat the frequency of the output 316 of the up-converter 312 in thetransceiver 224 is f_(c) minus the offset frequency, f_(o). For example,if f_(c)=1940 MHz and f_(o)=40 Hz, then the frequency output of theup-converter 312 in the transceiver 222 equal to 1940 MHz, the frequencyoutput of the up-converter 312 in the transceiver 220 is equal to1940.00004 MHz and the frequency output of the up-converter 312 in thetransceiver 224 is 1939.99996 MHz.

[0030] The signal strength of the pilot channel at any point in thesector is determined by the vector sum of all of the pilot channelsignals from each beam. For example, referring to FIG. 4A, the signal ata point from the second beam 110 is represented by vector 402. Thesignal at the same point from the first beam 108 is represent by vector404. Since the frequency of the signal transmitted on the first beam 108is offset by f_(o) from the frequency of the second beam 110, the vector404 rotates with respect to vector 402 and hence, the magnitude ofresultant vector 406 will fluctuate with a 1/f_(o) time period. FIG. 4Bshows a plot of the magnitude 408 of the result vector 406 versus time410. Due to the rotation of vector 404 a minimum 416 value occurs every1/f_(o) 414. In an IS-95 forward channel, the frame rate f_(f) is 50frames per second or a period of 20 ms. Also, each IS-95 frame isrepeated once. Therefore the offset frequency f_(o) is chosen such that1/f_(o) 414 is not a multiple of 1/f_(f) 412. This will prevent aminimum 416 from occurring at the same point in two consecutive framesthus significantly reducing the error rate.

[0031] Since the magnitude of the resultant vector 406 fluctuates with a1/f_(o) time period, f_(o) is chosen by empirical methods such that theoverall system performance is optimized. The optimum value of f_(o), foreach base station, is influenced by environmental factors, the maximumvelocity of the mobile stations, the frequency band and the over-the-airinterface. Typically f_(o) is greater than 30 Hz and less than 120 Hzfor a IS-95 CDMA communication system. Other over-the-air interfacestandards may have optimum performance at different values of f_(o).

[0032] The frequencies of oscillator 338 in transceiver 220, oscillator338 in transceiver 222 and oscillator 338 in transceiver 224 areidentical and chosen such that IS-95 signals at standard frequencies aredown-converted and demodulated.

[0033] The traffic channels on each beam are unique and uncorrelated sothat no cancellation of the traffic channels occurs.

[0034] In an alternative embodiment, the waveform of the oscillator 318in transceiver 222 is chosen such that the waveform of the output 316 ofthe up-converter 312 in the transceiver 222 is a standard IS-95 basestation transmit frequency, f_(c). The waveform of the oscillator 318 inthe transceiver 220 is chosen such that the waveform of the output 316of the up-converter 312 in the transceiver 220 is f_(c) with a timedependent phase offset within a range of −180° to 180°. The waveform ofthe oscillator 318 in the transceiver 224 is chosen such that thewaveform of the output 316 of the up-converter 312 in the transceiver224 is f_(c) with time dependent phase offset within a range of −180° to180°. The waveform of the output 316 of the up-converter 312 in thetransceiver 222 is the reference for 0° phase. The time dependent phaseoffset within may be sinusoidal, random or any other pattern thatresults in the phases of the output of oscillator 318 in the transceiver220, the output of oscillator 318 in the transceiver 222 and the outputof oscillator 318 in the transceiver 224 being incoherent. Hence, thephases of the first beam 108, the second beam 110 and the third beam 112are incoherent.

[0035] In the preferred embodiment the signals on input 226, input 230and input 234 of transceiver 220, transceiver 222 and transceiver 224respectively have identical IS-95 overhead channels (pilot,synchronization and paging channels) and unique IS-95 traffic channelscorresponding to mobile station(s) (not shown) that aretransmitting/receiving on the first beam 108, the second beam 110 andthe third beam 112 respectively. Mobile stations that move from beam tobeam or are in an area of overlapping beams are handled by IS-95 handoffprocedures.

[0036] In an alternative embodiment the signals on input 226, input 230and input 234 of transceiver 220, transceiver 222 and transceiver 224respectively have identical IS-2000 overhead channels and unique IS-2000traffic channels corresponding to mobile station(s) (not shown) that aretransmitting/receiving on the first beam 108, the second beam 110 andthe third beam 112 respectively. Mobile stations that move from beam tobeam or are in an area of overlapping beams are handled by IS-2000handoff procedures.

[0037] It should be noted that while an embodiment of the inventionusing a Butler matrix 240, as shown in FIG. 2A, does not require acalibration scheme to compensate for differential phases between thetransceivers, an embodiment using a digital beam former 260, as shown inFIG. 2B, does require a calibration scheme to compensate fordifferential phases between the transceivers.

[0038] Advantageously, the invention may be used with antenna systemsemploying diversity schemes, such as space diversity or polarizationdiversity. In all diversity schemes all overlapping beams should haveoffset frequencies or time dependent phase offsets.

[0039] While the preferred embodiment of the present invention has beendescribed and illustrated, it will be apparent to persons skilled in theart that numerous modifications and variations are possible. The scopeof the invention, therefore, is only to be limited by the claimsappended hereto.

1. An antenna system for a CDMA base station comprising: a plurality ofantennas defining a respective plurality of fixed beams which togethercover a sector and are connected to a beam-forming matrix; and aplurality of transmitters connected to the beam-forming matrix to drivethe plurality of antennas with signals including common overheadchannels, the transmitters being arranged to provide transmitfrequencies in a manner such that the transmit frequencies include anoffset from one another.
 2. The antenna system of claim 1 wherein thebeam-forming matrix is a Butler matrix.
 3. The antenna system of claim 1wherein the offset is chosen to be sufficient so as to reduceundesirable effects of signal cancellation.
 4. The antenna system ofclaim 3 wherein the reduction in undesirable effects of signalcancellation includes a reduction in error rate.
 5. The antenna systemof claim 1 wherein the signals are any CDMA communications standardformat that employs a redundant forward channel frame structure having aframe rate.
 6. The antenna system of claim 1 wherein the signals haveunique traffic channels.
 7. The antenna system of claim 5 wherein theoffset is a multiple other than that of the frame rate.
 8. The antennasystem of claim 1 wherein there are three antennas and threetransmitters.
 9. The antenna system of claim 1 wherein the offset isgreater than 30 Hz and less than 120 Hz.
 10. An antenna system for aCDMA base station comprising: a plurality of antennas defining arespective plurality of fixed beams which together cover a sector andare connected to a beam-forming matrix; and a plurality of transmittersconnected to the beam-forming matrix to drive the plurality of antennaswith signals including common overhead channels, the transmitters beingarranged to provide transmit phases in a manner such that the transmitphases include a time dependent offset from one another.
 11. The antennasystem of claim 10 wherein the beam-forming matrix is a Butler matrix.12. The antenna system of claim 10 wherein the offset is chosen to besufficient so as to reduce undesirable effects of signal cancellation.13. The antenna system of claim 12 wherein the reduction in undesirableeffects of signal cancellation includes a reduction in error rate. 14.The antenna system of claim 10 wherein the signals are any CDMAcommunications standard format that employs a redundant forward channelframe structure having a frame rate.
 15. The antenna system of claim 10wherein the signals have unique traffic channels.
 16. The antenna systemof claim 10 wherein the time dependent phase offsets are sinusoidal. 17.The antenna system of claim 10 wherein the time dependent phase offsetsare random.
 18. The antenna system of claim 10 wherein the timedependent phase offsets are any pattern that result in phases of thefixed beams being incoherent.
 19. The antenna system of claim 10 whereinthere are three antennas and three transmitters.
 20. An antenna systemfor a CDMA base station comprising: a plurality of antennas defining arespective plurality of fixed beams which together cover a sector andare connected to a beam-forming matrix; a plurality of transmittersconnected to the beam-forming matrix to drive the plurality of antennaswith signals including common overhead channels; and means in thetransmitters for providing transmit frequencies that include an offsetfrom one another.
 21. The antenna system of claim 20 wherein the offsetis chosen to be sufficient so as to reduce undesirable effects of signalcancellation.
 22. The antenna system of claim 20 wherein the signalshave unique traffic channels.
 23. An antenna system for a CDMA basestation comprising: a plurality of antennas defining a respectiveplurality of fixed beams which together cover a sector and are connectedto a beam-forming matrix; a plurality of transmitters connected to thebeam-forming matrix to drive the plurality of antennas with signalsincluding common overhead channels; and means in the transmitters forproviding transmit phases that include a time dependent phase offsetfrom one another.
 24. The antenna system of claim 23 wherein the offsetis chosen to be sufficient so as to reduce undesirable effects of signalcancellation.
 25. The antenna system of claim 23 wherein the signalshave unique traffic channels.
 26. In an antenna system for a CDMA basestation having a plurality of antennas defining a respective pluralityof fixed beams which together cover a sector, a method of applyingsignals including common overhead channels from a plurality oftransmitters comprising up-converting the signals in the transmitters toprovide a plurality of respective transmit frequencies wherein thetransmit frequencies are offset from one another.
 27. In an antennasystem for a CDMA base station having a plurality of antennas defining arespective plurality of fixed beams which together cover a sector, amethod of applying signals including common overhead channels from aplurality of transmitters comprising up-converting the signals in thetransmitters to provide a plurality of respective transmit phaseswherein the transmit phases have a time dependent offset from oneanother.
 28. A transceiver in an antenna system for a CDMA base stationcomprising a transmitter adapted to up-convert a signal including commonoverhead channels to provide a transmit frequency wherein the transmitfrequency is offset from a standard base station transmit frequency. 29.A transceiver in an antenna system for a CDMA base station comprising atransmitter adapted to up-convert a signal including common overheadchannels to provide a transmit phase wherein the transmit phase has atime dependent offset.
 30. An antenna system for a CDMA base stationcomprising: a digital beam former connected to a plurality oftransmitters; and a plurality of antennas defining a respectiveplurality of fixed beams which together cover a sector and are connectedto the plurality of transmitters to be driven with signals includingcommon overhead channels, the transmitters being arranged to providetransmit frequencies in a manner such that the transmit frequenciesinclude an offset from one another.
 31. The antenna system of claim 30wherein the offset is chosen to be sufficient so as to reduceundesirable effects of signal cancellation.
 32. The antenna system ofclaim 31 wherein the reduction in undesirable effects of signalcancellation includes a reduction in error rate.
 33. The antenna systemof claim 30 wherein the signals are any CDMA communications standardformat that employs a redundant forward channel frame structure having aframe rate.
 34. The antenna system of claim 30 wherein the signals haveunique traffic channels.
 35. The antenna system of claim 33 wherein theoffset is a multiple other than that of the frame rate.
 36. The antennasystem of claim 30 wherein there are three antennas and threetransmitters.
 37. The antenna system of claim 30 wherein the offset isgreater than 30 Hz and less than 120 Hz.
 38. An antenna system for aCDMA base station comprising: a digital beam former connected to aplurality of transmitters; and a plurality of antennas defining arespective plurality of fixed beams which together cover a sector andare connected to the plurality of transmitters to be driven with signalsincluding common overhead channels, the transmitters being arranged toprovide transmit phases in a manner such that the transmit phasesinclude time dependent offset from one another.
 39. The antenna systemof claim 38 wherein the offset is chosen to be sufficient so as toreduce undesirable effects of signal cancellation.
 40. The antennasystem of claim 39 wherein the reduction in undesirable effects ofsignal cancellation includes a reduction in error rate.
 41. The antennasystem of claim 38 wherein the signals are any CDMA communicationsstandard format that employs a redundant forward channel frame structurehaving a frame rate.
 42. The antenna system of claim 38 wherein thesignals have unique traffic channels.
 43. The antenna system of claim 38wherein the time dependent phase offsets are sinusoidal.
 44. The antennasystem of claim 38 wherein the time dependent phase offsets are random.45. The antenna system of claim 38 wherein the time dependent phaseoffsets are any pattern that result in phases of the fixed beams beingincoherent.
 46. The antenna system of claim 38 wherein there are threeantennas and three transmitters.
 47. An antenna system for a CDMA basestation comprising: a digital beam former connected to a plurality oftransmitters; and a plurality of antennas defining a respectiveplurality of fixed beams which together cover a sector and are connectedto the plurality of transmitters to be driven with signals includingcommon overhead channels; and means in the transmitters for providingtransmit frequencies in a manner such that the transmit frequencies thatinclude an offset from one another.
 48. The antenna system of claim 47wherein the offset is chosen to be sufficient so as to reduceundesirable effects of signal cancellation.
 49. The antenna system ofclaim 47 wherein the signals have unique traffic channels.
 50. Anantenna system for a CDMA base station comprising: a digital beam formerconnected to a plurality of transmitters; a plurality of antennasdefining a respective plurality of fixed beams which together cover asector and are connected to the plurality of transmitters to be drivenwith signals including common overhead channels; and means in thetransmitters for providing transmit phases that include a time dependentoffset from one another.
 51. The antenna system of claim 50 wherein theoffset is chosen to be sufficient so as to reduce undesirable effects ofsignal cancellation.
 52. The antenna system of claim 50 wherein thesignals have unique traffic channels.